The correct operation of a computer system usually presupposes that its physical memory performs correctly, such that each read at a physical memory address returns data content of precisely the same value that was last written to that memory address. Unfortunately, physical memory is subject to memory errors, such that this requirement is sometimes violated.
Physical memory errors come in at least two types. The first are transient errors that do not reflect a failing condition with the actual physical memory device, sometimes caused for example by cosmic rays or other transient conditions that “flip a bit”. The second are permanent errors arising from device failures, for example “shorting” a memory to always be stuck to a specific value, regardless of the value was written to it. Physical memory errors should be handled efficiently to maintain correct operation of the computer system.